The present invention is directed to an LED (light emitting diode) light source with a driving circuit to drive the LED light source. More particularly the present invention is directed to an LED light source with driving circuitry designed to maintain a constant temperature and therefore a constant useful life for the LED. The luminosity of the LED is adjusted to compensate for fluctuations in ambient temperature in order to prevent excessive depreciation in luminosity over time caused by overheating.
The amount of light produced by an electric light source decreases with time in a process known as lumen depreciation. For an LED, lumen depreciation is primarily caused by heat generated at the LED junction. The useful life of an LED is defined as the total time that the LED can be on before the luminosity decreases by a certain percentage, typically 40 percent. Therefore, the useful life of an LED is primarily temperature dependent.
In order to assign a useful lifetime to a particular LED, a maximum temperature must also be given. For example, an LED may have a lifetime of at least 20,000 hours if kept in an environment that remains below 30 degrees centigrade. This means the useful life may be significantly shorter if the LED is allowed to operate in an environment that regularly exceeds 30 degrees. Also, if kept in much lower temperatures, the LED may have a significantly longer lifetime.
In the prior art, LED circuits are well know with numerous examples and variations disclosed in U.S. Pat. No. 4,675,575. There is also an extensive variety of circuits specifically designed for driving LEDs with various functions and abilities. Several of these are disclosed in U.S. Pat. No. 7,116,294, and include methods for limiting current to the LEDs to prevent overheating. However, along with driving current, ambient temperature also plays an important role in LEDs overheating.
The method of using a temperature sensing means to control the luminous output of an LED is also known, as disclosed in U.S. Pat. Nos. 5,783,909 and 6,127,784. Both patents disclose the use of a temperature sensing means at an LED source to send feedback to a power supply. The average current supplied to the LED source is adjusted to maintain a constant LED luminosity by compensating for changes in luminosity resulting from changes in ambient temperature.
A circuit for maintaining a constant LED current is disclosed in U.S. Pat. No. 7,245,090 as well a method for determining the temperature of an LED based on the amount of current passing through it. However, like the above mentioned patents, the LED driver uses its temperature sensing capabilities to provide a constant luminous output rather than a constant temperature. This is important for many lighting applications where there is a minimum acceptable luminosity. In other applications such as household and decorative lighting, a constant luminosity may be less important than the longevity of the LED.
Therefore it would be desirable to have an LED light source with a driving circuit designed to maximize the useful life of the LED by sacrificing luminosity to compensate for high ambient temperature. Further, it would be desirable to have a LED light source that could reliably be assigned a useful lifetime that is independent of ambient temperature.